Conversion of 1,3-Propylene Glycol on Rutile TiO2(110)

The adsorption of 1,3-propylene glycol (1,3-PG) on partially reduced TiO2(110) and its conversion to products have been studied by a combination of molecular beam dosing and temperature-programmed desorption (TPD). When the Ti surface sites are saturated by 1,3-PG, similar to 80% of the molecules undergo further reactions to yield products that are liberated during the TPD ramp. In contrast to ethylene glycol (EG) and 1,2-propylene glycol (1,2-PG) that yield only alkenes and water at very low converages (<0.05 ML), two additional products, HCHO and C2H4, along with propylene (CH3CH=CH2) and water are observed for 1,3-PG. Identical TPD line shapes and desorption yields of HCHO and C2H4 suggest that these products result from C-C bond cleavage and are coupled. At higher 1,3-PG coverages (>0.1 ML), propanal (CH3CH2CHO) and two additonal products, 1-propanol (CH3CH2CH2OH) and acrolein (CH2=CHCHO), are observed. The desorption of 1-propanol is found to be coupled with the desorption of acrolein, suggesting that these products are formed by the disproportionation of two 1,3-PG molecules. The coverage-dependent TPD results further show that propylene formation dominates at low coverages (<0.3 ML), while the decomposition and disproportionation channels increase rapidly at higher coverages and reach yields comparable to that of propylene at the 1,3-PG saturation coverage of 0.5 ML. The observed surface chemistry clearly shows how the molecular structure of glycols influences their reaction pathways on oxide surfaces.